Driving circuit of light-emitting diode and driving method thereof

ABSTRACT

The present invention relates to a driving circuit of LED and a driving method thereof, which comprise a converting circuit and a driving unit. The converting circuit receives a pulse-width modulation (PWM) signal and generates an adjusting current according to the PWM signal. The driving unit receives the adjusting current and generates at least a driving current according to the adjusting current for driving a plurality of LEDs coupled to the driving unit. The magnitude of the driving current is adjusted according to the adjusting current. In addition, the driving unit has a bright-controlling pin used for receiving a control signal having a fixed level.

FIELD OF THE INVENTION

The present invention relates generally to a driving circuit and adriving method, and particularly to a driving circuit of light-emittingdiode and a driving method thereof.

BACKGROUND OF THE INVENTION

A light-emitting diode (LED) is a semiconductor electronic devicecapable of emitting light and composed by p- and n-type semiconductormaterials. It can generate self-radiating light in the ultravioletlight, visible light, and infrared light zones. Because an LED has theadvantages of saving energy, long lifetime, and high brightness, in thetrend of environmental protection, saving power, and reducing carbonemission, the applications of LED have become extensive increasingly,for example, traffic lights, streetlamps, flashlights, display devices,or lighting apparatuses.

Currently, most LED display devices, such as notebook computers or LCDpanels, output pulse width modulation (PWM) signals as the lightadjusting signals for LEDs using driving circuits. Thereby, the dutycycles of the switching signals can be adjusted and the LEDs areswitched on and off in high frequencies for achieving the purpose oftuning their brightness. Please refer to FIG. 1, which shows waveformsof PWM and switching signals according to the prior art. As shown in thefigure, when the PWM signal is high level, the switching signal S₁ turnson the LED by high-frequency switching; when the PWM signal is lowlevel, the switching signal S₁ is lowered the low level to turn off theLED. Because the LED is turned off when the PWM signal is low level andturned on only when the PWM signal is high level according to thismethod, the glitter phenomenon, which is hardly aware by human eyes butharmful to visual perception, will occur.

Although the switching frequency of the LED by the switching signal S₁may be high (approximately between 170 and 270 Hz) and people cannot beconscious of the glitter phenomenon, the ciliary muscles in human eyeswill respond to the glitter spontaneously. Therefore, staring at suchdisplay devices for a long time fatigues the ciliary muscles. Evenworse, it can cause the problems of high intraocular pressure andfeeling like vomiting.

Accordingly, the present invention provides a driving circuit for LEDand a driving method thereof, which solve the glitter problem in LEDs byoutputting a stable driving current to the LEDs.

SUMMARY

An objective of the present invention is to provide a driving circuit ofLED and a driving method thereof, which adjust the driving current ofLED according to the duty cycle of the PWM signal for adjusting thebrightness of LED.

For achieving the objective and effect described above, the presentinvention discloses a driving circuit of LED, which comprises aconverting circuit and a driving unit. The converting circuit receives aPWM signal and generates an adjusting current according to the PWMsignal. The driving unit receives the adjusting current and generates atleast one driving current according to the adjusting current for drivinga plurality of LEDs coupled to the driving unit. The magnitude of thedriving current is adjusted according to the adjusting current. Inaddition, the driving unit has a bright-controlling pin used forreceiving a control signal having a fixed level.

The present invention further discloses a driving method of LED, whichcomprises the steps of: transmitting a PWM signal to a convertingcircuit and generating an adjusting current according to the PWM signal;transmitting the adjusting current to a driving unit and generating atleast one driving current according to the adjusting current,. whereinthe driving current is used for driving a plurality of LEDs and themagnitude of the driving current is adjusted according to the adjustingcurrent; and transmitting a control signal having a fixed signal to abright-controlling pin of the driving unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows waveforms of PWM versus switching signals according to theprior art;

FIG. 2 shows a circuit diagram of the driving circuit of LED accordingto a preferred embodiment of the present invention;

FIG. 3 shows a circuit diagram of the driving unit according to apreferred embodiment of the present invention;

FIG. 4 shows a circuit diagram of the converting circuit according to apreferred embodiment of the present invention;

FIG. 5 shows a circuit diagram of the current control circuit accordingto a preferred embodiment of the present invention;

FIG. 6A shows waveforms of PWM versus switching signals according to apreferred embodiment of the present invention; and

FIG. 6B shows waveforms of PWM versus switching signals according toanother preferred embodiment of the present invention.

DETAILED DESCRIPTION

In order to make the structure and characteristics as well as theeffectiveness of the present invention to be further understood andrecognized, the detailed description of the present invention isprovided as follows along with embodiments and accompanying figures.

FIG. 2 shows a circuit diagram of the driving circuit of LED accordingto a preferred embodiment of the present invention. As shown in thefigure, the driving circuit 1 of LED according to the present inventioncomprises a converting circuit 10 and a driving unit 20. The convertingcircuit 10 receives a PWM signal S_(PWM), and produces an adjustingcurrent according to the PWM signal S_(PWM), as shown in FIG. 4. A setpin ISET of the driving unit 20 receives the adjusting current andproduces at least one driving current I_(D1)-I_(Dn) according to theadjusting current. The plurality of driving currents I_(D1)-I_(Dn) drivea plurality of LEDs 30 via a plurality of LED pins LED₁-LED_(n) of thedriving unit 20. The magnitude of the plurality of driving currentsI_(D1)-I_(Dn) is adjusted according to the adjusting current.

In addition, the driving unit 20 further comprises an input power pinVIN, a supply voltage pin VCC, a compensation pin COMP, a frequency setpin OSC, a light-tuning frequency set pin BOSC, an enable control pinEN, a bright-controlling pin DBRT, a gate control pin GATE, a currentsensing pin ISENSE, and a ground pin PGND.

The input power pin VIN receives an input voltage V_(IN) stabilized byan input capacitor C_(IN). The input voltage V_(IN) is furthertransmitted to the LED 30 via an inductor L, a diode D, and an outputcapacitor C_(OUT) and used as the power source for producing theplurality of driving currents I_(D1)-I_(Dn). A transistor M1 has a firstterminal, a second terminal, and a control terminal. The first terminalof the transistor M1 is coupled to the junction between the inductor Land the diode D. The second terminal of the transistor M1 is coupled tothe current sensing pin ISENSE. The control terminal of the transistorM1 is coupled to the gate control pin GATE for switching according to agate control signal V_(G) output by the gate control pin GATE. A sensingresistor R_(SENSE) is coupled between the current sensing pin ISENSE andthe ground pin PGND. The ground pin PGND is further coupled to a ground.When the transistor M1 is turned on, the sensing resistor R_(SENSE)converts the received current to a current sensing signal and outputsthe current sensing signal to the current sensing pin ISENSE.

A capacitor C_(VCC) is coupled between the supply voltage pin VCC andthe ground. A capacitor C_(COMP) is connected in series with a resistorR_(COMP) and coupled between the compensation pin COMP and the ground. Aresistor R_(OSC) is coupled between the frequency set pin OSC and theground. A resistor R_(BOSC) receives a supply voltage V_(DD) and iscoupled to the light-tuning frequency set pin BOSC. A capacitor C_(BOSC)is coupled between the light-tuning frequency set pin BOSC and theground. The enable control pin EN is used for receiving an enable signalENS and enabling the driving unit 20. The bright-controlling pin DBRTreceives a control signal, and the control signal has a fixed level.According to this embodiment of the present invention, the fixed levelof the control signal is high level.

Besides, because the level of the enable signal ENS received by theenable control pin EN is high level and the level of the control signalreceived by the bright-controlling pin DBRT is also high level, when theenable signal ENS is transmitted to the enable control pin EN, it can betransmitted to the bright-controlling pin DBRT simultaneously. Thereby,when the enable signal ENS is used as the control signal of thebright-controlling pin DBRT, it is not necessary to generate anothersignal, simplifying the circuit complexity and further shrinking thecircuit area. Nonetheless, the present invention is not limited to thecase where the control signal is just the enable signal ENS; the controlsignal can be another high-level signal.

Please refer to FIG. 3, which shows a circuit diagram of the drivingunit according to a preferred embodiment of the present invention. Asshown in the figure, the driving unit 20 comprises a current controlcircuit 200, a plurality of oscillators 210, 220, a plurality ofcomparators 230, 240, 250, 260, a feedback control unit 270, anoperational unit 280, a logic control unit 290, and a voltage stabilizer300. The current control circuit 200 is coupled between the set pin ISETand the plurality of LED pins LED₁-LED_(n) and coupled to the convertingcircuit 10 via the set pin ISET then coupled to the plurality of LEDs 30via the plurality of LED pins LED₁-LED_(n). The current control circuit200 is used for producing the plurality of driving currentsI_(D1)-I_(Dn). according to the adjusting current generated by theconverting circuit 10.

The oscillator 210 is coupled between the light-tuning frequency set pinBOSC and a negative input of the comparator 230 and outputs asaw-toothed signal STS₁ to the negative input of the comparator 230. Apositive input of the comparator 230 is coupled to bright-controllingpin DBRT for receiving the enable signal ENS. The comparator 230compares the enable signal ENS and the saw-toothed signal STS₁ andoutputs a switching signal SW for switching and producing the pluralityof driving currents I_(D1)-I_(Dn). Because the enable signal ENSmaintains at the high level as the driving circuit 1 is operatednormally and maintains at the low level as the driving circuit 1 stopsoperating, the switching signal SW output by the comparator 230 ismaintained at the high level continuously when the driving circuit 1 isoperated normally, such that the current control circuit 200 can keepproducing the plurality of driving currents I_(D1)-I_(Dn). Thereby, thesituation of the driving currents reducing to the low level for a longerperiod of time and resulting in the glitter phenomenon as seen in theprior art will not occur.

A positive input of the comparator 250 is coupled to the current sensingpin ISENSE; a negative input of the comparator 250 is coupled to theground pin PGND and the ground. The comparator 250 compares the voltagelevels between the current sensing signal and the ground, outputting acomparing signal to the operational unit 280. The feedback control unit270 is used for detecting the output signal of the current controlcircuit 200 and outputting a feedback signal to a positive input of thecomparator 240. A negative input of the comparator 240 receives acomparing voltage V_(com). The comparator 240 outputs a compensatingsignal to the compensation pin COMP after comparing the feedback signaland the comparing voltage V_(com), and which is used for compensatingthe circuit. The oscillator 220 is coupled among the frequency set pinOSC, the operational unit 280, and the logic control unit 290, andoutputs a saw-toothed signal STS₂ to the operational unit 280 and thelogic control unit 290. The operational unit 280 operates the saw-toothsignal STS₂ and the comparing signal and outputs to a positive input ofthe comparator 260. A negative input of the comparator 260 receives thecompensation signal. The comparator 260 compares the compensation signaland the operating result output by the operational unit 280 and outputs.Meanwhile, the logic control unit 290 generates the gate control signalV_(G) according to the output of the comparator 260 and the saw-toothsignal STS₂. The frequency of the gate control signal V_(G) correspondsto the switching signal SW.

The voltage stabilizer 300 is coupled to the input power pin VIN, thesupply voltage pin VCC, and the ground, and a supply voltage V_(C) isgenerated at the supply voltage pin VCC according to the input voltageV_(IN) received at the input power pin VIN.

Please refer to FIG. 4, which shows a circuit diagram of the convertingcircuit according to a preferred embodiment of the present invention. Asshown in the figure, the converting circuit 10 comprises a plurality ofresistors R1, R2, a voltage dividing circuit 102, and a switch 100. Theresistor R1 has a first terminal and a second terminal. The firstterminal of the resistor R1 is coupled to the set pin ISET of thedriving unit 20 for receiving a reference voltage V_(VREF1) output bythe driving unit 20. The second terminal of the resistor R1 is coupledto the ground. In addition, a predetermined current I_(PC)is generatedat the resistor R1. The magnitude of the predetermined current I_(PC) isdetermined according to the reference voltage V_(REF1) and theresistance of the resistor R1, and giving the following equation (1).The voltage dividing circuit 102 comprises a plurality of voltagedividing resistors R3, R4. A first terminal of the voltage dividingresistor R3 is coupled to the supply voltage pin VCC of the driving unit20 for receiving the supply voltage V_(C). A first terminal of thevoltage dividing resistor R4 is coupled to a second terminal of thevoltage dividing resistor R3; a second terminal of the voltage dividingresistor R4 is coupled to the ground. The voltage dividing resistors R3,R4 are connected in series, and a reference voltage V_(REF2) isgenerated after providing the supply voltage Vc.

$\begin{matrix}{I_{PC} = \frac{V_{{REF}\; 1}}{R\; 1}} & (1)\end{matrix}$

The resistor R2 has a first terminal and a second terminal. The firstterminal of the resistor R2 is coupled to the set pin ISET and the firstterminal of the resistor R1 and receives the reference voltage V_(REF1).The second terminal of the resistor R2 receives the reference voltageV_(REF2). An adjusting current I_(A) is generated at the resistor R2.The magnitude of the adjusting current I_(A) is determined according tothe reference voltage V_(REF1), the reference voltage V_(REF2), and theresistance of the resistor R2, and giving the following equation (2).Moreover, a reference current I_(REF) output by the set pin ISET of thedriving unit 20 is equal to the sum of the adjusting current I_(A) andthe predetermined current I_(PC), and giving the following equation (3).

$\begin{matrix}{I_{A} = \frac{V_{{REF}\; 1} - V_{{REF}\; 2}}{R\; 2}} & (2) \\{I_{REF} = {I_{A} + I_{PC}}} & (3)\end{matrix}$

The switch 100 has a first terminal, a second terminal, and a controlterminal. The first terminal of the switch 100 is coupled to the secondterminal of the resistor R3; the second terminal of the switch 100 iscoupled to the ground; and the control terminal of the switch 100receives the PWM signal S_(PWM). The duty cycle of the PWM signalS_(PWM) is used for determining the conduction time of the switch 100and thus further controlling the voltage level of the reference voltageV_(REF2). When the PWM signal p_(PWM) is low level, the switch 100 iscut off. Then the voltage level of the reference voltage V_(REF2) isproduced by dividing the supply voltage V_(c) using the resistors R3,R4. On the other hand, when the PWM signal S_(PWM) is high level, theswitch 100 is turned on. The node A is coupled to the ground via theswitch 100 and lowering the voltage level of the node A. Namely, thevoltage level of the reference voltage V_(REF2) is lowered accordingly.Consequently, the voltage difference between the reference voltageV_(REF1) and the reference voltage V_(REF2) is increased, whichincreases the adjusting current I_(A) and gives the following equation(4). In the equation (4), D is the percentage of high-level PWM signalS_(PWM) during each period, that is, the duty cycle.

$\begin{matrix}{V_{{REF}\; 2} = {\frac{V_{CC}*R\; 4}{{R\; 3} + {R\; 4}}*\left( {1 - D} \right)}} & (4)\end{matrix}$

According to the above description, the voltage level of the referencevoltage V_(REF2) is determined according to the duty cycle of the PWMsignal S_(PWM) of this embodiment and hence changing the magnitude ofthe adjusting current I_(A). Because the reference current I_(REF) isthe sum of the adjusting current I_(A) and the predetermined currentI_(PC), the magnitude of the reference current I_(REF) is alsodetermined by the duty cycle of the PWM signal S_(PWM). As thepercentage of the duty cycle is higher, the value of the referencecurrent I_(REF) is higher; as the percentage of the duty cycle is lower,the value of the reference current I_(REF) is lower.

In addition, the converting circuit 10 can further comprise voltagedividing resistor R5, R6, a diode D1, and/or a voltage stabilizingcapacitor C1. The voltage dividing resistor R5 receives the PWM signalS_(PWM) and is coupled to the control terminal of the switch 100. Thevoltage dividing resistor R6 is coupled between the control terminal ofthe switch 100 and the ground. The voltage dividing resistors R5, R6 areused for dividing the voltage of the PWM signal S_(PWM) and outputtingto the control terminal of the switch 100. The diode D1 has a positiveterminal and a negative terminal. The positive terminal of the diode D1is coupled to the second terminal of the voltage dividing resistor R3;the negative terminal of the diode D1 is coupled to the first terminalof the voltage dividing resistor R4. In the present invention, the diodeD1 is regarded as an ideal diode used for preventing reverse flow ofcurrent. Thereby, there is no voltage drop when the diode D1 is forwardbiased and turned on, making the voltage level of the node A equal tothe voltage level of the reference voltage V_(REF2). The voltagestabilizing capacitor C1 has a first terminal and a second terminal. Thefirst terminal of the voltage stabilizing capacitor C1 is coupled to thesecond terminal of the resistor R2; the second terminal of the voltagestabilizing capacitor C1. The voltage stabilizing capacitor C1 is usedfor stabilizing the voltage level of the reference voltage V_(REF2).

Please refer to FIG. 5, which shows a circuit diagram of the currentcontrol circuit according to a preferred embodiment of the presentinvention. As shown in the figure, the current control circuit 200comprises a plurality of current generating circuit 202, 206 and acurrent mirror 204. The current generating circuit 202 is coupled to theset pin ISET of the driving unit 20, and generates the reference currentI_(REF) according to the adjusting current I_(A) and the predeterminedcurrent I_(PC). Besides, as shown in FIG. 4, the set pin ISET of thedriving unit 20 is coupled to the converting circuit 10. The currentmirror 204 is coupled to the current generating circuit 202 and mirrorsthe reference current I_(REF) for producing a mirror current I_(M). Thecurrent generating circuit 206 is coupled to the current mirror 204 andthe LEDs 30, and generating the driving currents I_(D1)-I_(Dn) accordingto the mirror current I_(M) for driving the plurality of LEDs 30. Forconvenience, according to the present embodiment, the current controlcircuit 200 is coupled to an LED 30 only and the current generatingcircuit 206 generates a driving current I_(Dn). Nonetheless, inpractice, the current control circuit 200 can be coupled to a pluralityof LEDs 30 and generating a plurality of driving currents I_(D1)-I_(Dn)as shown in FIG. 2.

The current generating circuit 202 comprises an operational amplifier2020 and a transistor 2022. A positive input of the operationalamplifier 2020 receives an input voltage V_(in), and a negative input ofthe operational amplifier 2020 is coupled to the set pin ISET of thedriving unit 20. In addition, since the voltage levels of the positiveand negative inputs are identical because of the characteristics of theoperational amplifier, the voltage level of the negative input of theoperational amplifier 2020 is equal to the input voltage V_(in), whichis transmitted to the converting circuit 10 via the set pin ISET andused as the reference voltage V_(REF1), as shown in FIG. 4. Thetransistor 2022 is coupled between the current minor 204 and the set pinISET and controlled by an output of the operational amplifier 2020.Thereby, the reference current I_(REF) flows through the transistor2022.

The current generating circuit 206 comprises a plurality of resistors2060, 2064, an operational amplifier 2062, and a transistor 2066. Theresistor 2060 is coupled to the current minor 204, receiving the minorcurrent I_(M), and the minor current I_(M) is converted to a conversionvoltage V_(CV). A positive input of the operational amplifier 2062 iscoupled to the resistor 2060 and receives the conversion voltage V_(CV).The resistor 2064 is coupled between a negative input of the operationalamplifier 2062 and the ground, and converts the conversion voltageV_(CV) to the driving current I_(Dn). The transistor 2066 is coupledbetween the LED 30 and the resistor 2064 and controlled by an output ofthe operational amplifier 2062. The driving current I_(Dn) is generatedat the resistor 2064 and flows through the transistor 2066.

According to the above equations (1), (2), and (3), the followingequation (5) is deduced. In addition, in the framework in FIG. 5 ofconverting the reference current I_(REF) to the driving current I_(Dn),the driving current I_(Dn) can be set to be identical to the referencecurrent I_(REF) by setting the current minor 204 and the resistors 2060,2064. Nonetheless, the present invention is not limited to the case thatthe driving current I_(Dn) should be identical to the reference currentI_(REF).

$\begin{matrix}{I_{REF} = {{I_{A} + I_{PC}} = {1000*\left( {\frac{V_{{REF}\; 1} - V_{{REF}\; 2}}{R\; 2} + \frac{V_{{REF}\; 1}}{R\; 1}} \right)}}} & (5)\end{matrix}$

Moreover, the driving circuit 1 can further comprises an output switch50 and/or a protection circuit 60. The output switch 50 is coupledbetween the transistor 2066 and the LED 30, controlled by the switchingsignal SW or the gate control signal V_(G) for providing the drivingcurrent I_(Dn) to the LED 30. The protection circuit 60 is coupled tothe set pin ISET of the driving unit 20, and generates a protectionsignal PS to stop the operation of the driving circuit 1 when the setpin ISET is short circuit detected.

Please refer to FIGS. 3, 4, 5, 6A, and 6B concurrently. FIG. 6A showswaveforms of PWM versus switching signals according to a preferredembodiment of the present invention; and FIG. 6B shows waveforms of PWMversus switching signals according to another preferred embodiment ofthe present invention.

As shown in the figure, because the bright-controlling pin DBRT if thedriving unit 20 receives the always-high enable signal ENS as thecontrol signal and the comparator 230 generates the switching signal SWafter comparing the enable signal ENS and the saw-toothed signal STS₁for controlling turning on or off of the LED 30, the LED 30 is switchedaccording to the PWM signal S_(PWM). As shown in FIGS. 6A and 6B, nomatter the duty cycle of the PWM signal S_(PWM) is 20% or 80%, both ofthe switching signal SW and the gate control signal keep turning on theLED 30 continuously and thus making the driving current I_(Dn) flowthrough the LED 30 continuously. Thereby, there will be no glitterphenomenon.

In addition, in the converting circuit 10 according to the presentinvention, if the resistance of the resistor R1 is 61 KΩ, the resistanceof the resistor R2 is 15.25 KΩ, the resistance of the voltage dividingresistor R3 is 2.78 KΩ, the resistance of the voltage dividing resistorR4 is 1.22 KΩ, the voltage level of the supply voltage V_(C) is 5V, thevoltage level of the reference voltage V_(REF1) will be 1.22V.

Thereby, according to the embodiment in FIG. 6A, the duty cycle of thePWM signal S_(PWM) is 20%. From the equation (4), the voltage ofV_(REF2) is 1.22V. Besides, from the equation (5), the driving currentI_(Dn) is 20 mA. According to the embodiment in FIG. 6B, from theequation (4), the voltage level of V_(REF2) is 0.305V; from the equation(5), the driving current I_(Dn) is 80 mA.

Accordingly, the converting circuit 10 according to the presentinvention can convert the PWM signal S_(PWM) to the adjusting currentI_(A) varied according to the duty cycle of the PWM signal S_(PWM).Thereby, the current control circuit 200 of the driving unit 20 adjuststhe magnitude of the driving current according to the adjusting currentI_(A), and hence adjusting the brightness of the LED 30. Consequently,according to the present invention, because the adjustment of thebrightness of the LED 30 is not necessarily done by adjusting theturn-on and turn-off of the LED 30, the glitter phenomenon can beavoided.

In summary, the driving circuit of LED according to the presentinvention comprises a converting circuit and a driving unit. Theconverting circuit can convert the PWM signal to the adjusting currentI_(A) varied according to the duty cycle of the PWM signal. Thereby, thedriving unit can adjust the magnitude of the driving current accordingto the adjusting current and hence adjusting the brightness of the LED.Consequently, according to the present invention, because the adjustmentof the brightness of the LED is not necessarily done by adjusting theturn-on and turn-off of the LED 30, the glitter phenomenon can beavoided.

Accordingly, the present invention conforms to the legal requirementsowing to its novelty, nonobviousness, and utility. However, theforegoing description is only embodiments of the present invention, notused to limit the scope and range of the present invention. Thoseequivalent changes or modifications made according to the shape,structure, feature, or spirit described in the claims of the presentinvention are included in the appended claims of the present invention.

The invention claimed is:
 1. A driving circuit of light-emitting diode,comprising: a converting circuit, receiving a pulse width modulationsignal (PWM), and producing an adjusting current according to said pulsewidth modulation signal; and a driving unit, receiving said adjustingcurrent, producing at least one driving current according to saidadjusting current for driving a plurality of light-emitting diodescoupled to said driving unit, and the magnitude of said driving currentis adjusted according to said adjusting current; wherein said drivingunit has a bright-controlling pin used for receiving a control signalhaving a fixed level; and wherein said converting circuit comprises: afirst resistor, having a first terminal and a second terminal, saidfirst terminal of said first resistor coupled to a set pin of saiddriving unit for receiving a first reference voltage, said secondterminal of said first resistor coupled to a ground; a second resistor,having a first terminal and a second terminal, said first terminal ofsaid second resistor coupled to said set pin of said driving unit andsaid first terminal of said first resistor for receiving said firstreference voltage, said second terminal of said second resistorreceiving a second reference voltage, wherein the magnitude of saidadjusting current is determined according to said first referencevoltage, said second reference voltage, and said second resistor; and aswitch, having a first terminal, a second terminal, and a controlterminal, said first terminal of said switch receiving said secondreference voltage, said second terminal of said switch coupled to saidground, said control terminal of said switch receiving said pulse widthmodulation signal, and the duty cycle of said pulse width modulationsignal determining the conduction time of said switch and controllingthe voltage level of said second reference voltage.
 2. The drivingcircuit of claim 1, wherein said converting circuit further comprises avoltage dividing circuit, receiving a supply voltage and coupled to saidground, and dividing said supply voltage and producing said secondreference voltage.
 3. The driving circuit of claim 1, wherein saidconverting circuit produces a predetermined current at said firstresistor; the magnitude of said predetermined current is determinedaccording to said first reference voltage and said first resistor; andsaid driving unit produces said driving current according to saidadjusting current and said predetermined current.
 4. The driving circuitof claim 1, wherein said converting circuit further comprises a voltagestabilizing capacitor, having a first terminal and a second terminal,said first terminal of said voltage stabilizing capacitor coupled tosaid second terminal of said second resistor, and said second terminalof said voltage stabilizing capacitor coupled to said ground forstabilizing said second reference voltage.
 5. The driving circuit ofclaim 1, wherein said driving unit comprises: a first current generatingcircuit, coupled to a set pin of said driving unit, producing areference current according to said adjusting current, and said set pincoupled to said converting circuit; a current mirror, coupled to saidfirst current generating circuit, and producing a mirror currentaccording to said reference current; and a second current generatingcircuit, coupled to said current mirror and said plurality oflight-emitting diodes, and producing said driving current according tosaid mirror current for driving said plurality of light-emitting diodes.6. The driving circuit of claim 5, wherein said first current generatingcircuit comprises: an operational amplifier, having a positive input, anegative input, and an output, said positive input receiving an inputvoltage, said negative input coupled to said set pin of said drivingunit; and a transistor, coupled between said current and said set pin ofsaid driving unit, and controlled by said output of said operationalamplifier to make said reference current flow through said transistor.7. The driving circuit of claim 5, wherein said second currentgenerating circuit comprises: a first resistor, coupled to said currentmirror, and converting said mirror current to a conversion voltage; anoperational amplifier, having a positive input, a negative input, and anoutput, and said positive input coupled to said first resistor forreceiving said conversion voltage; a second resistor, coupled betweensaid negative input of said operational amplifier and a ground; and atransistor, coupled between said plurality of light-emitting diodes andsaid second resistor, and controlled by said output of said operationalamplifier to make said driving current flow through said transistor. 8.The driving circuit of claim 1, and further comprising an output switch,coupled between said driving unit and said plurality of light-emittingdiodes, and controlled by a switching signal for providing said drivingcurrent to said plurality of light-emitting diodes.
 9. The drivingcircuit of claim 1, and further comprising a protection circuit, coupledto a set pin of said driving unit, and controlling said driving circuitto stop operating when said set pin is short circuit detected.
 10. Adriving method of light-emitting diode, comprising the steps of:transmitting a pulse width modulation signal to a converting circuit,and producing an adjusting current according to said pulse widthmodulation signal; transmitting said adjusting current to a drivingunit, producing at least one driving current according to said adjustingcurrent for driving a plurality of light-emitting diodes, and themagnitude of said driving current is adjusted according to saidadjusting current; and transmitting a control signal having a fixedlevel to a bright-controlling pin; wherein said step of “producing anadjusting current according to said pulse width modulation signal”comprises the steps of: transmitting a first reference voltage to afirst terminal of a first resistor and a first terminal of a secondresistor within said converting circuit; transmitting a second referencevoltage to a second terminal of said second resistor, and determiningthe magnitude of said adjusting current according to said firstreference voltage, said second reference voltage, and said secondresistor; and determining the conduction time of a switch according tothe duty cycle of said pulse width modulation signal and controlling thevoltage level of said second reference voltage.
 11. The driving methodof claim 10, and further comprising a step of transmitting a supplyvoltage to a voltage dividing circuit of said converting circuit anddividing said supply voltage for producing said second referencevoltage.
 12. The driving method of claim 10, and further comprisingsteps of: producing a predetermined current at said first resistoraccording to said first reference voltage, and determining the magnitudeof said predetermined current according to said first reference voltageand said first resistor; and producing said driving current according tosaid adjusting current and said predetermined current.
 13. The drivingmethod of claim 10, wherein said step of “transmitting said adjustingcurrent to a driving unit, producing at least a driving currentaccording to said adjusting current for driving a plurality oflight-emitting diodes” comprises: transmitting said adjusting current toa first current generating circuit of said driving unit, and producing areference current according to said adjusting current; transmitting saidreference current to a current mirror of said driving unit, andproducing a mirror current according to said reference current; andtransmitting said mirror current to a second current generating circuitof said driving unit, and producing said driving current according tosaid mirror current for driving said plurality of light-emitting diodes.14. The driving method of claim 13, wherein said step of “transmittingsaid adjusting current to a first current generating circuit of saiddriving unit, and producing a reference current according to saidadjusting current” comprises: transmitting an input voltage to apositive input of an operational amplifier; and switching a transistoraccording an output of said operational amplifier to make said referencecurrent flow through said transistor.
 15. The driving method of claim13, wherein said step of “transmitting said reference current to acurrent mirror of said driving unit, and producing a mirror currentaccording to said reference current” comprises: transmitting said mirrorcurrent to a first resistor, and converting said mirror current to aconversion voltage; transmitting said conversion voltage to a positiveinput of an operational amplifier, and outputting said conversionvoltage via a negative input of said operational amplifier; transmittingsaid conversion voltage to a second resistor; and switching a transistoraccording an output of said operational amplifier to make said secondresistor produce said driving current according to said conversionvoltage and said reference current flow through said transistor.
 16. Thedriving method of claim 10, further comprising: switching an outputswitch according a switching signal for providing said driving currentto said plurality of light-emitting diodes.
 17. The driving method ofclaim 10, further comprising: detecting a set pin of said driving unit,and producing a protection signal to stop said driving circuit when saidset pin is short circuit.